Electromechanical locking devices are used in security and access control arrangements for buildings. One such locking device, commonly known as a “drop-bolt”, is intended for fitting in the door frame above the top of a door. The device includes an electrically activated bolt which is moveable between an extended position, where it engages with a recess in the top of the door to lock the door, and a retracted position, where the bolt is withdrawn from the recess in the door, allowing the door to be opened.
There is a problem with drop-bolts caused by a condition known as “preload”. That is, when the bolt is extended, and there is some sideways pressure on the door, such as caused by a person leaning on the door, or due to differing air pressures on either side of the door, then the locking device may be unable to withdraw the bolt, and so the door is not able to be unlocked.
Some have tried to overcome the problem of preload by way of locking devices which utilise a roller nut that is driven by a screw shaft. The roller nut engages with a pivotal bolt to rotate the bolt to an extended position. However, such devices are typically complex in their construction and involve a large number of moving parts. Locks with large numbers of moving parts can be expensive to manufacture and the large number of parts introduce opportunities for component failure, and provide potential opportunities for intruders to compromise the lock.